Field of the Invention
This invention relates to a turbocharger for an internal combustion engine. More particularly, this invention relates to a mixed-flow turbocharger with variable turbine geometry.
Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power density without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of reduced emissions.
Turbochargers include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a bearing housing connecting the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft rotatably supported in the center bearing housing connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it increases the air mass flow rate, airflow density, and air pressure delivered to the engine's cylinders via the engine's intake manifold.
The flow of exhaust gas to the turbine wheel, which drives rotation of the turbine wheel, can be generally characterized as either radial-flow or mixed-flow. In a radial-flow turbocharger, the exhaust gas is directed to the turbine wheel through a wheel inlet of the turbine housing in a radial direction relative to the axis of rotation. In other words, the flow of exhaust gas through the wheel inlet to the turbine wheel in radial-flow turbochargers is perpendicular to the axis of rotation of the turbine wheel. In contrast, in a mixed-flow turbocharger, the exhaust gas is directed to the turbine wheel through the wheel inlet of the turbine housing in a direction that includes both radial and axial components relative to the axis of rotation. In other words, the flow of exhaust gas through the wheel inlet to the turbine wheel in mixed-flow turbochargers is non-perpendicular to the axis of rotation of the turbine wheel.
To improve efficiency, responsiveness, or the operating range of the turbocharger, it is often advantageous to regulate the exhaust gas flowing to the turbine wheel. One method of regulating the exhaust gas flowing to the turbine wheel is commonly referred to by several names, including Variable Turbine Geometry (VTG), Variable Geometry Turbine (VGT), Variable Nozzle Turbine (VNT), or simply Variable Geometry (VG). VTG turbochargers include a plurality of adjustable guide vanes positioned within the wheel inlet leading to the turbine wheel and pivotally supported between upper and lower vane rings. The space between adjacent guide vanes constitutes flow channels for the exhaust gas flowing to the turbine wheel and the geometry of the flow channels is adjustable by adjusting the guide vanes within a pre-determined range of angular positions between an open position and a closed position. In the open position, the guide vanes extend generally radially relative to the axis of rotation to allow the exhaust gas to flow through the wheel inlet to the turbine wheel. In the closed position, the guide vanes extend generally tangentially relative to the axis of rotation to block the exhaust gas from flowing through the wheel inlet to the turbine wheel. In order to provide a high boost pressure at low engine speeds, the guide vanes are adjusted to constrict the flow channels between adjacent guide vanes. This results in the exhaust gas moving through the flow channels at a high speed. The increased kinetic energy of the exhaust gas is transferred to the turbine wheel, increasing the boost pressure. At high engine speeds, the guide vanes are adjusted to open up the flow channels between adjacent guide vanes. This results in the exhaust gas impacting the turbine wheel at a lower speed, thus decreasing the boost pressure.
In radial-flow turbochargers, each guide vane pivots about a shaft axis that is parallel to the axis of rotation of the turbine wheel. Therefore, clearance between the guide vanes and the upper and lower vane rings remains consistent as the guide vanes pivot between the open and closed positions. In addition, a control ring having an axis of rotation that is parallel to the shaft axis of each guide vane includes actuator blocks that are secured to the control ring and are operatively coupled with a vane fork of each guide vane to actuate the guide vanes between the open and closed positions in response to rotation of the control ring. It is appreciated that the vane forks pivot in a plane that is parallel to the plane of rotation of the control ring such that there is sliding contact between the vane forks and actuator blocks during adjustment of the guide vanes.
It is well known in the field of turbochargers that mixed-flow turbochargers overcome several inherent limitations of radial-flow turbochargers and therefore provide a better utilization of the engine's exhaust gas energy. However, incorporating guide vanes into mixed-flow turbochargers presents several problems. First, the wheel inlet is oriented to direct the flow of exhaust gas to the turbine wheel in a non-perpendicular direction relative to the axis of rotation of the turbine wheel. As such, sides of the upper and lower vane rings facing the wheel inlet define conical surfaces extending around the axis of rotation of the turbine wheel. In other words, the wheel inlet is a conical frustum shape. Therefore, clearance between the guide vanes and the upper and lower vane rings is not consistent as the guide vanes pivot between the open and closed positions. More specifically, in the open or generally radially extending position, clearances between the guide vanes and the upper and lower vane rings are uniform along the length of the guide vanes, from a leading edge to a trailing edge. In contrast, in the closed or generally tangentially extending position, the conical surface of the upper vane ring curves away from the leading and trailing edges of the guide vanes. As such, there is increased clearance between the upper vane ring and the leading and trailing edges of the guide vanes, which reduces the performance and efficiency of the turbocharger. Also in the closed position, the conical surface of the lower vane ring curves toward the leading and trailing edges of the guide vanes. As such, there is decreased clearance between the lower vane ring and the leading and trailing edges of the guide vanes, which must be accounted for to prevent binding of the guide vanes.
Second, each guide vane pivots bout a shaft axis that is non-parallel to the axis of rotation of the turbine wheel. In other words, the shaft axis of each guide vane is arranged at an acute angle in an inclined manner with respect to the axis of rotation of the turbine wheel. Therefore, the shaft axis of each guide vane is non-parallel to the axis of rotation of the control ring. Consequently, the vane forks pivot in planes that are non-parallel to the plane of rotation of the control ring such that there is rolling contact between the vane forks and the actuator blocks during adjustment of the guide vanes. As such, a typical vane fork and actuator block arrangement that is used in radial-flow turbochargers will bind if used in mixed-flow turbochargers.
It is desirable, therefore, to provide a mixed-flow turbocharger with adjustable guide vanes wherein clearances between the guide vanes and upper and lower vane rings remain consistent during adjustment of the guide vanes between open and closed positions.